This invention relates to position finely adjusting apparatus which is used to accurately position a workpiece which is required to be worked precisely, for example. Such accurate positioning apparatus are required for precisely aligning the positions of a mask and a wafer that are used to prepare large integrated circuits by concurrent exposure of electron beams, X-rays or ion bombardments for accurately positioning the mask for forming a pattern thereon.
In recent years, the density of the large integrated circuit is increasing with the result that the accuracy of forming a pattern on a wafer should be increased to an order of 0.1 micron or less from an order of microns required by conventional transistor integrated circuits.
Such highly accurate working is not possible with conventional photoetching technique due to diffraction of the exposure light and other reasons. Accordingly, the photoetching technique has been replaced to a large extent by a scanning type electron microscope system utilizing an electron beam having a high resolution and a long focal distance. However, according to this system, a pattern is formed on a wafer by utilizing an extremely fine electron beam so that its working time is long and such system is not suitable for mass production. One example of the scanning type electron microscope system is described in D. R. Heriott, R. J. Collier et al. paper "EBES; a Practical Electron Lithographic System" on pages 385 to 392 of IEEE Taransaction Electron Device, July, 1975, Vol. ED-22, No. 7. To solve this problem it has been proposed to use an improvedmethod wherein a scanning type electron microscope system having a high working accuracy is used to firstly prepare a master pattern, and then concurrently expose and bake a pattern on a wafer by using the master pattern and an electron beam, ultraviolet ray, X-ray or ion bombardment working technique thereby improving productivity. With this method, the accuracy of forming the pattern on the wafer is determined by the accuracy of the mechanism utilized to align the relative position of the mask and the wafer. To attain this object, it is necessary to use a feed adjusting mechanism having a stepping accuracy of at least 0.01 micron or less. Since, patterns are generally baked repeatedly on the same wafer about 6 to 8 times, when one takes into consideration an accumulated error of the positioning operations it is necessary to improve the accuracy of feeding to 0.03 micron (3 .times. 10.sup.-6 cm) or more.
According to one method of determining the relative position between a mask and a wafer, as shown in FIG. 1 a pluraity of reference marks 13 and 14 are formed on the mask 11 and the wafer 12, respectively, and the relative positions of the wafer and mask are adjusted in the X and Y axes directions as well as in the rotational direction (angle .theta.) by positioning means such that each light beam or X-ray passes through aligned reference marks 13 and 14. The light beams are received by light receivers 16 to control the positioning means. To this end, the positioning means is required to finely adjust the position of the mask 11 or wafer 12. Further, the accuracy of positioning must satisfy the value described above in three directions.
In the case of the concurrent exposure method, in order to limit as far as possible the spreading of the electron beams or X-rays so as to improve the baking accuracy of the pattern, it is necessary to decrease as far as possible the spacing between the wafer and the mask and to maintain them in exact parallel relationship. Especially, in the concurrent X-ray exposure for a plurality of chips on the wafer, the relative spacing should be several microns. Further, in mass production it is difficult to maintain the wafer and mask in exact parallel relationship so that the positioning mechanism is required to be finely adjustable in the Z axis direction in addition to the X and Y axis directions and rotation .theta..
There was no positioning mechanism that can satisfy these requirements, but the mechanism shown in FIG. 2 may have some possibility. One example of such mechanism is described in Offenlegungs Schrift: ff 1772314, published in Germany.
The positioning mechanism shown in FIG. 2 comprises a rectangular table 20 for supporting a wafer 12. At four corners of the table 20 are formed projections 20a, 20b, 20c and 20d for securing cylindrical piezoelectric elements 21 through 24, respectively, which are driven to align the reference mark 14 on the wafer 12 with the reference mark 13 on the mask 11, such alignment being made by adjusting the relative position in the directions X, Y and .theta. under the supervision of the light beam detectors 25. The range of adjustment of this mechanism is about 10 microns so that in order to position beforehand the wafer in this range of adjustment, a highly accurate microstage is combined with the positioning mechanism.
With this positioning mechanism where piezoelectric elements 21 and 23 in the direction X are driven for moving the wafer 12 in the direction X, since piezoelectric elements in the direction Y are also connected to the table 20, the table will be influenced by the mechanical impedance of these elements so that it is extremely difficult to move straightly the wafer 12 in the direction of X axis. Similar difficulty also occurs when the piezoelectric elements 23 and 24 in the direction Y are driven. This means that it is difficult to position the wafer through a desired path. Accordingly, it is not only impossible to move the wafter to the absolutely correct position but also impossible to adjust the movement of the wafer to a next predetermined position with reference to a selected point. Accordingly, it is extremely difficult to adjust the position at a high accuracy of the order of 0.1 micron. For the same reason, this mechanism cannot move the table 20 in a straight line in each direction for detecting the marks on the wafer and the mask. Accordingly, when one tries to move the wafer 12 obliquely by simultaneously driving it in X and Y directions the wafer moves along a complicated path that cannot be controlled readily. Further undesirable strains are applied to the wafer 12, table 20 and piezoelectric elements 21 through 24 and in the extreme case, the piezoelectric elements would be ruptured. When the mechanical coupling between the piezoelectric elements and the table 20 is decreased for the purpose of preventing damage to the piezoelectric elements the coupling becomes loose or may cause a backlash. Due to the low sensitivity of the cylindrical piezoelectric elements utilized in this mechanism, in order to realize a range of adjustment of the order of 10 microns it is necessary to increase the length of the cylindrical elements to about several 10s cm, or to decrease the diameter to about 0.3 mm or to increase the driving voltage of the piezoelectric elements (in terms of electric field, higher than 3000 V/cm or more). It is difficult to make long and thin the piezoelectric elements due to the avaialable space and mechanical strength. On the other hand when the driving voltage is increased the hysteresis characteristic inherent to the piezoelectric element becomes excessive thus rendering it more difficult to perform accurate control. For these reasons, with the mechanism shown in FIG. 2, it has been almost impossible to perform position adjustment with an accuracy of less than 0.1 micron.
FIG. 3 shows another example of a prior art position adjusting mechanism described in French Pat. No. 2178640. In this mechanism, X and Y stages 26 and 27 are movably stacked on a base plate 25. The stages 26 and 27 are driven in X and Y axes by a motor not shown and a coarse positioning is done by a laser interferometer 28. As shown, Y stage 27 is mounted on X stage 26 and a microtable 30 driven by an electromagnet 29 is supported on the Y stage 27 by means of leaf springs 31. Fine adjustments of the position is provided by utilizing the balance between the attractive force of the electromagnet 29 and the reaction of the leaf springs 31.
With this mechanism, however, since the relationship between the attractive force of the electromagnetic and the body to be attracted thereby, namely the table 30, is nonlinear, fine position adjustments in the X and Y directions are also nonlinear. Accordingly, the control system for effecting such adjustment is extremely complicated. Especially, when the control speed is increased the control becomes unstable due to its nonlinear nature. By the construction shown in one embodiment of said French patent it is impossible to make high the resonance frequency. Moreover since a stacked construction is used the motion of the mechanism is unstable. Also in this construction, mutual interference between the driving forces in respective directions is inevitable. Although such mechanism can be used to form a pattern of low density, it cannot be used to effect at a high speed and at a high accuracy the alignment of the positions of the mask and wafer for the purpose of increasing the productivity.
In order to use the linear portion only of the control characteristic for the purpose of eliminating the problem caused by the nonlinear characteristic the distance between the electromagnet and the body attracted thereby may be used. However, as the attractive force decreases in proportion to the square of the distance, it is necessary to increase the size of the electromagnet or to increase the current flowing therethrough. This of course increases the size of the apparatus.
The mechanism utilizing an electromagnet, or an electromagnetic transducer has a large magnetic leakage and cannot provide an accurate control having an accurcy of less than 0.1 micron due to the mechanical or electrical interference between transducers for respective axes. Especially, when a vibration is applied for detecting the marks in the concurrent exposure system it is extremely difficult to align the relative position of the mask and the wafer where the mutual interference caused by the magnetic leakage exists.
In order to eliminate the effect of the magnetic leakage it is necessary to sufficiently separate the transducers for avoiding the mutual interference therebetween, thereby increasing the size of the mechanism.
Magnetostrictive or electrostrictive electromechanical transducers may be used for eliminating inherent problems of the electromagnetic transducers, but the former cannot be used for performing the position adjustment at high accuracies due to its low sensitivity, whereas the latter presents problems similar to those of the mechanism shown in FIG. 2 so that it cannot be used.
Even when the use of the prior art mechanisms described above is limited to a mere feed control and when the feed plane is limited to one plane including X and Y axes and angle .theta. it is impossible to operate with accuracies of less than 0.1 micron and at high speeds. Further, the use of a vibration for correctly detecting the position alignment of the wafer and the mask is almost impossible. Admitting that a fine position alignment is possible with the prior art mechanism (neglecting the defect of decreasing the accuracy) the range of the possible adjustment of the prior art mechanism is about 5 microns. Further, a high accuracy microstage is necessary to preset the accuracy in this range. This not not only requires a large and expensive mechanism but also requires a multistate control, thus increasing the cost of the mechanism. Where extremely high accuracies are required for not only X, Y and .theta. directions but also for Z axis direction as in the case of the X-ray concurrent exposure such mechanism can never be used.